Contaminant-free high resistivity water is critical to the fabrication of integrated circuits. Point-of-use filters are designed as the last opportunity to remove contaminants from the water used in integrated circuit manufacture. A point-of-use filter processes fluid which is to be utilized immediately in a localized manufacturing step. The manufacture of integrated circuits involves hundreds of steps in which silicon wafers are repeatedly exposed to processes such as lithography, etching, doping, and deposition of metals. Between these processing steps, numerous cleaning steps are also essential and they are accomplished through direct contact of the wafer with ultrapure water. Throughout all of these steps, the semiconductive nature of the silicon and its surface must be maintained and/or specifically controlled. Contamination can alter the semiconductive nature of the silicon or disturb the intended circuit design, thereby reducing the yield of integrated circuits. Point-of-use filters used for high resistivity water must, therefore, remove microparticulates without adding contaminants, i.e., exhibit low levels of ionic and total organic carbon (TOC) extractables. In addition, the effluent water from point-of-use filters must approach the level of purity of the influent as quickly as possible.
During the past decade, the microelectronics industry has advanced through miniaturization. Within the semiconductor industry it is believed that particles of more than one tenth of the line geometry on a microchip are capable of causing a defect. (See M. Yang and D. Tolliver, "Ultrapure Water Particle Monitoring for Advanced Semiconductor Manufacturing," Journal of Environmental Science, July/August, 1989.) The production of 4Mb chips with a minimum circuit feature size of 0.8 micron is imminent. (See R. J. Kopp, "Forecast 1991: Timing Is Key," Semiconductor International, January, 1991.) Particles as small as 0.1 micrometer may, therefore, lead to failure of a semiconductor element. A particle can prevent the completion of a line or a particle can bridge across two lines. Contamination can be either direct on the silicon surface or it may be a contamination of a masking surface, changing the circuit design which is printed.
Micro particles that contaminate high resistivity water are often generated in the distribution piping and tool plumbing of the circulation loop. As a result, point-of-use filters must be capable of retaining very fine particulates, such as cellular debris and pyrogens. Colloidal and oligomeric silica are known to pass through point-of-use filters and cause contamination. There silicon compounds are negatively charged in high purity deionized (DI) water. The result is that the silicon wafer attracts these silica particles. Oligomeric silica is known to contaminate water which is considered free of particles as fine as 0.1 micrometer. (See R. Iscoff, "Water Purity for the DRAM Generation," Semiconductor International, January, 1991). Dissolved contaminants such as humic acid, surfactants, and bacterial metabolites which pass through the filter to the silicon chips can also result in yield loss.
A filter membrane can achieve fluid clarification by different mechanisms. Particulate material can be removed through mechanical sieving wherein all particles larger than the pore diameter of the filter membrane are removed from the fluid. With this mechanism, filtration efficiency is controlled by the relative size of the contaminant and filter pore diameter. Accordingly, the efficient removal of very small particles, e.g., less than 0.1 micrometer in diameter, requires filter membranes with very small pore sizes. Such fine pore filter membranes tend to have the undesirable characteristics of high pressure drop across the filter membrane, reduced dirt capacity, and shortened filter life.
A filter may also remove suspended particulate material by adsorption onto filter membrane surfaces. Removal of particulate material by this mechanism is controlled by the surface characteristics of (1) the suspended particulate material and (2) the filter membrane. Most suspended solids which are commonly subjected to removal by filtration are negatively charged in aqueous systems. This feature has long been recognized in water treatment processes where cationic flocculating agents, oppositely charged to the suspended matter, are employed to improve settling efficiencies during water clarification through flocculation.
Colloid stability theory can be used to predict the interactions of electrostatically charged particles and surfaces. If the charges of suspended particle and the filter membrane surface are of like sign and with zeta potentials of greater than about 20mV, mutual repulsive forces will be sufficiently strong to prevent capture by adsorption. If the zeta potentials of the suspended particle and the filter membrane surface are small or, more desirably, of opposite sign, particles will tend to adhere to the filter membrane surfaces with high capture efficiencies. Microporous filter membranes characterized by positive zeta potentials are capable of removing negatively charged particles much smaller than the pore diameters of the membrane through the mechanism of electrostatic capture. Such membranes have potential applicability in the microelectronics industry since it is known that most particles encountered as contaminants in industrial practice have a negative zeta potential.
While membranes with a positive zeta potential offer significant advantages for retention of microparticulates, it is critical that these filters do not inadvertently introduce contaminants downstream of the filter. Extractables from point-of-use filters are a major concern in the microelectronics industry. Extractables are substances which may be potentially released from a filter element and contaminate its effluent. If such contaminants are deposited on silicon wafers, they cause a defect, resulting in a yield loss during the microchip fabrication process. As a result, industry practice is to test the resistivity of the effluent at the point-of-use filters. Only after the effluent has reached the level of purity of influent can the microchip washing and rinsing steps begin. The semiconductor industry requires deionized water having a resistivity approaching the theoretical maximum for water at 25.degree. C., 18.3 megohm-cm. Current industry practice requires water having a minimum resistivity in the range of 17.8 to 18.1 megohm-cm. The production of water having resistivity of greater than 18 megohm-cm is complex, time consuming, and expensive. Therefore, effluent water flow from point-of-use filters must approach the level of purity of the influent as quickly as possible. In order to do this, the filters must not only retain particulate matter but they must also have a very low level of extractable material.
Ionic extractables, especially sodium, are worrisome to the semiconductor industry. Minuscule amounts of ionic species in ultrapure water can cause dramatic reductions in integrated circuit yield. Even very low concentrations, which would not show up in a resistivity test, can react with the ultrapure silicon wafer, thereby doping the silicon in an unwanted fashion.
Organic extractables must also be kept to a minimum. These are generally measured as total organic carbon (TOC). It is believed that organic extractables are absorbed on the surface of the wafer, causing defective crystallization during high temperature processing and in epitaxial growth. (See N. Hashimoto, K. Satou, T. Shinoda, K. Takino, "Manufacturing Equipment For Ultrapure Water For 16M Devices," Proceedings From The Ninth Annual Semiconductor Pure Water Conference, January, 1990.) Extremely low concentrations of TOC will not noticeably affect the effluent resistivity reading but may nonetheless be detrimental to yield output.
Nylon membrane filters having a positive charge have gained acceptance as point-of-use filters for ultrapure water. Nylon membranes which have been charge modified by a coating process involving amines and epoxide group-containing compounds have been suggested for use as point-of-use filters for ultrapure water. However, filters utilizing these membranes suffer from high levels of ionic extractables, such as sodium and chloride. As a result, their usefulness in the microelectronics industry is limited.
The membrane disclosed in U.S. Pat. No. 4,702,840 is a positively charged nylon membrane which has been prepared by cocasting the nylon polymer with a quaternary ammonium group-containing polymer. Filters utilizing these membranes have low extractables and high retention efficiencies but require cost prohibitive processing steps in order to manufacture filter elements that rinse up quickly in 18.2 megohm-cm ultrapure water. Since the microelectronics industry standards for ultrapure water have become more stringent there is a need for an improved filter which utilizes a positively charged microporous polymeric membrane that has high retention efficiency, low extractables, and a quick rinse up time in 18.2 megohm-cm ultrapure water.